1. Field of the Invention
The present invention relates to the field of radiant wall burners. In particular the invention relates to radiant wall burners wherein a number of technologies are combined in a single burner arrangement so as to achieve low NOx and low noise.
2. The State of the Prior Art
Reduction and/or abatement of NOx in radiant burners has always been a desirable aim. Some NOx abatement has been achieved in the past by staging a portion of the gaseous fuel. Low pressure staged gas may be introduced into the combustion zone either from low pressure gas tips arranged around the periphery of the burner or from a center gas tip which protrudes through the center of the end cap of the radiant burner nozzle. These arrangements have not always been successful because, for NOx abatement purposes, the staged fuel should not be introduced into areas of the combustion zone where the oxygen concentration is greater than about 4% by volume.
Various problems encountered in prior art burners are addressed by the concepts and principles of the present invention. In particular, the invention addresses the ever present need for NOx abatement. In accordance with one aspect of the invention, it has been found that when gas is burned in a staged manner it may sometimes be responsible only for about 6 ppm (vol) of the total NOx emissions of an individual burner. Accordingly it has been thought to be desirable to adapt the concept of fuel staging to radiant wall burners. Several different configurations have been tried, some more successful than others, but none with complete satisfaction. In some configurations, staged fuel has been delivered through a plurality of tubes at very low pressure around the circumference of the burner. In such a case the staged fuel is introduced in proximity to a combusting mixture which is still quite rich in oxygen. This excess oxygen leads to higher flame temperatures and higher NOx content in flue gases.
In other configurations, staged gas has been introduced into the combustion zone from the axially distal end of the premix discharge nozzle. This configuration, where the staged fuel is injected coaxially at the center line of the premix burner assembly, has been somewhat more successful in achieving lower NOx emissions than the first configuration discussed above, at least in part due to the fact that the introduction point is located in spaced relationship to the face of the tile as well as away from the oxygen rich stream leaving the premix discharge nozzle. The down side of this particular methodology is that the momentum of the staged gas jet can and often does pull the primary oxygen rich premixed stream into the jet as an entrained flow thereby increasing the availability of excess oxygen as well as the production of NOx. This problem is exacerbated in applications requiring a multiplicity of individual burners in an array because of the interactions between burners.
In accordance with an important aspect of the invention, a low NOx burner nozzle assembly is provided for a radiant wall burner. The assembly includes an elongated hollow burner tube and a discharge nozzle. The burner tube has a central, longitudinally extending axis and defines a conduit extending along the axis for supplying a mixture of fuel and air to a radiant combustion area of a combustion zone that extends radially and surrounds the nozzle assembly. This mixture may desirably be fuel lean. The discharge nozzle is mounted on the tube at a downstream end of the conduit adjacent the combustion zone, and the same is adapted for receiving the mixture of fuel and air from the conduit and directing the same into the radiant combustion area in an essentially radial direction relative to the axis of the tube. The discharge nozzle may include a plurality of flow directing members arranged in an array which extends circumferentially around the discharge nozzle and the members may desirably be arranged to define therebetween a plurality of passageways which extend in a generally radial direction relative to the axis. The discharge nozzle may also include an end cap that is mounted on the members in a position to close the conduit and prevent flow of the mixture in a direction along the axis. Thus, the mixture is caused to flow through the passageways in a generally radial direction.
Preferably, the flow directing members may be arranged so that some of the passageways therebetween have a larger flow area than others. Desirably, the members may be in the form of plates which are essentially rectangular in shape. Ideally, the passageways may also extend in an axial direction. In a much preferred form of the invention, the end cap may have a lateral edge which is located at a first radial distance from the axis, and the members may each have an outer edge located at a second radial distance from the axis. The second radial distance ideally may be greater than the first radial distance such that passageways defined by the members extend radially outward beyond the lateral edge of the end cap.
In accordance with another preferred form of the invention, the nozzle may include an internal baffle positioned and arranged to redirect at least aportion of the mixture flowing through the conduit and cause the same to flow through the passageways in a generally radial direction.
In yet another preferred form of the invention, the end cap may have an axially extending hole therein, and the nozzle assembly may include a centrally located staged fuel burner nozzle made up, for example, of a length of tubing which extends along the axis of the conduit. The assembly may also include a staged burner nozzle tip at a downstream end of the length of tubing. In accordance with this aspect of the invention, the staged fuel burner nozzle may desirably be arranged so as to protrude axially through the hole. Importantly, the tip ideally may have a fuel delivery orifice therein for delivering fuel to the combustion zone in spaced relationship to the radiant combustion area.
In one desirable form of the invention, the delivery orifice may be disposed so as to introduce fuel gas into zone 20 at an upward and outward angle relative to a plane that is perpendicular to the axis. Preferably, the angle may be at least about 30xc2x0, and for some purposes in accordance with the invention, the delivery orifice may be disposed to introduce fuel gas in a direction along the axis.
Even more desirably, the staged fuel burner nozzle may be positioned such that a downstream portion of the length of tubing protrudes beyond the end cap so that the tip is positioned in axially spaced relationship relative to the end cap. Ideally, in this particularly desirable form of the invention, the low NOx burner nozzle may include an elongated protective sheath disposed in surrounding relationship to the protruding portion of the length of tubing and the tip. Such sheath may desirably include an opening disposed in alignment with the orifice. The sheath may also be provided with one or more vent openings configured to permit gases between the sheath and the length of tubing to escape into the combustion zone. In accordance with the foregoing aspects of the invention, the staged burner nozzle may be of significant value, regardless of the form of the discharge nozzle. Thus, the staged burner tip of the invention may be used with any sort of radial discharge nozzle that operates to spread a combustible mixture of fuel and air radially across the face of a radiant tile.
In accordance with yet another aspect of the invention, the burner tube may comprise a venturi tube having a throat that is in communication with an air supply and a source of fuel gas under pressure. The venturi tube may desirably be arranged such that the flow of fuel gas through the throat induces a flow of air from the air source whereby the mixture of fuel and air is created in the throat and caused to flow toward the discharge nozzle.
The invention also provides a low NOx radiant wall burner comprising a burner tile having a central opening surrounded by a radiant tile face and an elongated low NOx burner nozzle assembly as described above that extends through such opening. The face of the burner tile may be either dished or flat.
In addition, the invention provides a method for operating a burner comprising providing a mixture of fuel and air at a centrally located point adjacent a face of a burner tile, separating the mixture into a plurality of separate streams and causing such streams to flow radially outwardly from the centrally located point across the face of the tile, and causing the velocity of some of the streams to be greater than the velocity of others of the streams.
The invention further provides a method for operating a burner which includes the steps of providing a mixture of fuel and air at a centrally located point adjacent a face of a burner tile, separating the mixture into a plurality of separate streams and causing the streams to flow radially outwardly from the point across the face of the tile, causing the streams to combust to form flames, each having an outer peripheral terminus spaced radially from the point, and providing secondary air to the flame at a location adjacent the termini.
In yet another form, the invention provides a method for operating a burner that comprises providing a mixture of fuel and air, causing the mixture to flow along a path to a centrally located point adjacent a face of a burner tile, separating the mixture into a plurality of separate streams and causing the streams to flow radially outwardly from the path across the face of the tile, causing the streams to combust to form flames in an area of a combustion zone adjacent the face, and providing staged fuel to the zone at a location spaced from the area. In accordance with this form of the invention, the oxygen content of the gases at the location where the staged fuel is introduced is desirably not more than about 4% by volume.
The invention also provides a low NOx burner assembly which includes an elongated hollow burner tube providing a longitudinally extending conduit for supplying a mixture of fuel and air to a combustion zone. The burner tube has an outer wall surrounding the conduit, a longitudinally extending central axis and a pair of spaced ends. The assembly also includes a discharge nozzle at one of the ends of the burner tube, an inlet for a mixture of fuel and air at the other end of the burner tube, and at least one port extending through the wall at a location between the discharge nozzle and the inlet to communicate the conduit with an external area located outside the burner tube. Desirably the port may have a center axis which is essentially perpendicular to the central axis of the tube. Alternatively, the port may have a center axis which is at an angle relative to the central axis of the tube. Ideally the assembly may include a plurality of ports extending through the wall of the tube at respective locations between the discharge nozzle and the inlet. In one preferred form of the invention, the ports may be arranged in one or more rows which extend around the outer wall of the tube.
In another form of the invention, the ports described above may be utilized in combination with a discharge nozzle that includes a plurality of flow directing members as described, which are arranged to define therebetween a plurality of passageways which extend in generally radial directions relative to said axis, and an end cap mounted on said members in a location to redirect at least a portion of the mixture flowing from the end of the conduit and cause the same to flow through said passageways in a generally radial direction. In accordance with the invention, the members may be arranged so that some of the passageways have a larger flow area than others of the passageways.
The nozzle assembly having at least one port extending through the wall of the burner tube may be used as a component of a low NOx radiant wall burner that includes a burner tile having a central opening. In such a case, the nozzle assembly may extend through the central opening of the tile. Desirably, the discharge nozzle may include a plurality of flow directing members which are arranged to define therebetween a plurality of passageways which extend in generally radial directions relative to the axis of the burner tube, and an end cap mounted on said members in a location to redirect at least a portion of the mixture flowing from the end of the conduit and cause the same to flow through said passageways in a generally radial direction so that when ignited, the redirected mixture of fuel and air provides a generally laterally extending flame having an outer peripheral extremity at a location in said zone spaced radially from said axis.
The invention further provides a method for operating a burner which includes the steps of causing a mixture of fuel and air to flow toward a centrally located point adjacent a face of a burner tile, causing additional air to flow toward a location adjacent said face which is spaced laterally from said point, and separating a portion of said mixture and intermixing the same with said additional air to create an ultra lean admixture capable of flameless oxidation before the additional air reaches said location. More particularly, the method may include the steps of causing a mixture of fuel and air to flow toward a centrally located point adjacent a face of a burner tile, separating a first portion of said mixture into a plurality of separate streams and causing said streams to flow radially outwardly from said point across the face of said tile, causing said streams to combust to form flames, each having an outer peripheral terminus spaced radially from said point, providing secondary air to said flame at a location adjacent said termini, adding a second portion of said mixture to said secondary air at a location upstream from said location to create an admixture capable of flameless oxidation at the face of said tile, and flamelessly oxidizing said admixture at said face to create relatively cool oxidation products. In accordance with the concepts and principles of the invention, oxidation products may be admixed with the combusting gases to thereby dilute and cool the same. In further accordance with the principles and concepts of the invention, a flow of recirculated flue gas may be provided to said flame at a location adjacent said termini.
Prior art burners of the premix type of design have not been able to utilize as many NOx abatement technologies in a single burner as are provided in the burner arrangements of the present invention.